Direct methanol fuel cell (DMFC) systems directly supply methanol fuel and oxygen (as an oxidant) to a stack and generate electricity by means of the resulting electrochemical reaction. The stack, which substantially generates electricity in the DMFC systems, has at least one unit cell stacked thereon, which includes a membrane electrode assembly (hereinafter, referred to as MEA) and a separator (or bipolar plate). The MEA includes an anode, a cathode, and an electrolyte membrane interposed between the anode and the cathode. Each of the anode and cathode includes a fuel diffusion layer for supply and diffusion of fuel, a catalyst layer in which oxidation/reduction reactions of fuel occur, and an electrode supporter. The catalyst layer is typically made of noble metal, such platinum. A catalyst made of an alloy of a transition metal, such as ruthenium, rhodium, osmium, or nickel, may be used for the anode, in order to prevent catalyst poisoning caused by carbon monoxide, which is a reaction by-product. The electrode supporter is made of carbon paper or carbon fabric and is waterproofed so as to easily supply fuel and discharge reaction products. The electrolyte membrane is a polymer membrane having a typical thickness of 50-200 μm. Particularly, the electrolyte membrane is a hydrogen ion exchange membrane, which contains moisture and has ion conductivity.
Electrode reactions occurring at the stack of DMFC systems include an anode reaction, by which supplied fuel oxidizes, and a cathode reaction, by which oxygen in the supplied air reacts with hydrogen ions from the anode and reduces. More particularly,
a. anode reaction:CH3OH+H2O→CO2+6H++6e−  (reaction formula 1)
b. cathode reaction:3/2O2+6H++6e−→3H2O  (reaction formula 2)
c. overall reaction:CH3OH+3/2O2→2H2O+CO2  (reaction formula 3)
At the anode, where the oxidation reaction (reaction formula 1) occurs, carbon dioxide, hydrogen ions, and electrons are created by a reaction between methanol and water, and the created hydrogen ions are transmitted to the cathode via the electrolyte membrane. At the cathode, where the reduction reaction (reaction formula 2) occurs, water is created by a reaction among the hydrogen ions, electrons transmitted via an external circuit, and oxygen. In the overall reaction (reaction formula 3) of DMFC systems, methanol and oxygen react with each other and create water and carbon dioxide. Particularly, one molecule of methanol reacts with oxygen and creates two molecules of water.
The fuel supplied to the anode is typically not pure methanol, but a mixture of water and methanol adjusted to a predetermined concentration. When high-concentration methanol is used, crossover occurs via the electrolyte membrane (i.e. fuel passes through the ion exchange membrane). This degrades the generation performance of the fuel cell. Therefore, it is customary to use low-concentration methanol of 0.5-2M (2-8 vol %).
Therefore, for DMFC systems, methanol fuel of a predetermined concentration is continuously supplied to the anode, and, to this end, pure methanol and water, which is collected from the cathode, are supplied to a fuel mixing device. In addition, air is continuously supplied to the cathode. In conventional DMFC systems, a pump is used to supply pure methanol, methanol fuel, and water, and the amount of supply is adjusted by controlling the capacity and number of pumps. For the air supply, the DMFC systems generally use a pump or a blower.
However, conventional DMFC systems have a problem in that it is difficult to monitor in real-time whether or not methanol, for example, is continuously supplied during operation. For example, monitoring an interruption in the fuel supply due to a problem of the conduit that connects the pump to the stack while the pump is running properly can be difficult. Particularly, in the case of a DMFC system used in a mobile communication device or laptop computer, the diameter of the conduit, through which fuel flows, is typically reduced due to the small size of the system. This increases the possibility of a problem in the conduit and, as a result, makes it harder to monitor such a problem. Furthermore, the same problem may occur in conduit carrying air.